The present invention relates to integrated circuit (semiconductor) memory devices and more specifically, to phase-change memory devices and methods for forming the same.
Semiconductor memory devices are generally classified as volatile memory devices or as non-volatile memory devices, based on whether data can be maintained when power to the device is turned off. Examples of a volatile memory device include a dynamic random access memory (DRAM) or a static random access memory (SRAM). An example of a non-volatile memory device is a FLASH memory. In such memory devices, stored binary information, having a “0” or a “1” state, may be determined by sensing a stored charge in a memory cell.
Investigations continue to develop new types of memory devices having, for example, a non-volatile property, temporary accessing, a low power operation property and/or high integration. An example of one such memory type being investigated is a phase-change memory device. A phase-change memory device generally operates using a phase-change material. A crystal state of the phase-change material may be changed using resistive heating, which may be provided using a current pulse. A chalcogenide compound of germanium Ge, antimony Sb and tellurium Te may be used as the phase-change material.
Thus, chalcogenides are a class of material that may be used to store information in an integrated circuit memory device. Chacogenide material may be electrically stimulated to change states, from an amorphous state to an increasingly crystalline state. In the amorphous state, chacogenide material generally exhibits a high electrical resistivity. As a chalcogenide material progresses into an increasingly crystalline state, its electrical resistivity generally decreases.
In chacogenide-based memories, the memory cells are typically formed by disposing chalcogenide material between two electrodes. A size of a contact area between the electrode and the chalcogenide material appears to be related to the operating speed of device, the power requirement of the device and/or performance of the device. When heat is applied to the chalcogenide material through the contact area to the electrodes, a portion of the chalcogenide material (referred to as a program volume) generally changes state. For a smaller program volume, a smaller program current may generally be used for changing state. The size of program volume is typically related to the size of the contact area. To reduce the program current and/or time, studies have been directed to reducing the size of the contact area.
FIG. 1 is a schematic cross-sectional view of a conventional phase-change memory device. As shown in FIG. 1, the device include insulation layers 11, 19, a bottom electrode 13, a phase-change material layer 15, an upper electrode 17, an upper electrode contact 21 and a bit line 23. The bottom electrode 13 is shown as a plug-type electrode that provides a current pulse to change a crystalline state of the phase-change material layer 15. For the conventional phase-change memory device illustrated in FIG. 1, the bottom electrode 13 and the phase-change material layer 15 are in contact at a flat contact area 25. The size of the contact area 25 depends on the configuration and/or diameter of the plug-type bottom electrode 13. When current flows through the bottom electrode 13, the current generally passes through the phase-change material layer 15 and the upper electrode 17. For the configuration illustrated in FIG. 1, the current is concentrated on the contact area 25 as schematically illustrated by the arrows. The arrows further indicate that the current may spread from the contact area 25 through the whole phase-change material layer 15. As such, the current density generally gradually decreases. Therefore, as illustrated in FIG. 1 by the shaded region, a program volume having a hemisphere configuration may be formed on the contact area 25.
In the conventional phase-change memory device of FIG. 1, it is generally required to reduce the size of the contact area 25 to reduce program current. The reduction in contact area 25 may be provided by reducing the diameter of the bottom electrode 13. However, the diameter of the bottom electrode 13 may be limited by a resolution of a photolithography process used in forming the bottom electrode 13. As a result, there is generally a limit to how much the program current may be reduced by this approach.
In an alternative approach, U.S. Pat. No. 6,329,666 describes a bottom electrode having a tip shape to reduce the program current. FIG. 2 illustrates a phase-change memory device as described in U.S. Pat. No. 6,329,666. As shown in FIG. 2, the device includes a semiconductor substrate 100, a bottom electrode 102 and a tip 114 of the bottom electrode 102. The device of FIG. 2 also includes insulation layers 116, 124, a phase-change material layer 120, an upper electrode 122, 128, an interconnection 126 and a program volume 130. The tip 114 includes a top surface 118. In the device of FIG. 2, a dimension of the top surface 118 of the contact area is formed smaller than in the device of FIG. 1 by using a tip 114 on the bottom electrode 102. As a result, the program current may be decreased. However, the contact area between the bottom electrode 102 and the phase-change material layer 120, that is, the contact area defined by the top surface 118, is flat. Accordingly, as with the device shown in FIG. 1, the configuration of the program area 130 is hemispherical. In other words, when current flows through the bottom electrode 102, the current passes through the phase-change material layer 120 and flows to the upper electrode 128, 122. As such, the current may be concentrated on the contact area 118 and then spread through the whole phase-change material layer 120. As a result current density typically gradually decreases.
For both the known phase-change memory devices described above, current flows from the contact area to the phase-change material layer in a variety of directions as shown by the arrows in FIG. 1 and FIG. 2. This generally arises from the flat (planar)) contact area. As a result, current density generally decreases as the current passes through the phase-change material layer, which may increase the difficulty of reducing the program current in such devices.